1. Field of the Invention
The present invention relates to a Sn-based metal-coated steel strip having a beautiful surface state without uneven luster, and being excellent in appearance.
2. Description of the Related Art
Because Sn-based metal-coated steel strips are excellent in corrosion resistance and can be produced at low cost, they have been widely used as components of various products used in corrosive environments. Therefore, many Sn-based metal-coated steel strips excellent in corrosion resistance and many processes for producing the same have heretofore been proposed.
Japanese Unexamined Patent Publication (Kokai) No. 7-207841 discloses a low-reflecting highly corrosion-resistant coated metal materials having a two-phase Sn—Zn metal coating containing at least 15 wt. % of Sn and at least 65 wt. % of Zn. The coated metal materials include a coated metal material having a thin nickel layer and a two-phase Sn—Zn metal coating formed thereon. The coated metal materials are used as architectural materials having corrosion resistance and good color tone.
The specification of U.S. Pat. No. 5,491,035 (published in 1996) discloses a coated steel strip excellent in corrosion resistance and coated with a two-phase Sn—Zn alloy layer, by hot dip coating, on an optionally formed nickel layer. The coated steel strip has a non-lustrous color tone on the surface and is used as an architectural material.
Architectural materials, and architectural external materials in particular, have recently been required to have no uneven luster on their surfaces in order to ensure good appearance and color tone, or to ensure a good color tone and good aesthetic appearance. However, suppressing the manifestation of the uneven luster in the above architectural materials has never been considered.
Japanese Unexamined Patent Publication (Kokai) No. 8-269733 discloses, as a Sn-based metal-coated steel strip for other applications, a highly corrosion-resistant rust-preventive steel strip for fuel tanks wherein a Sn—Zn alloy coating layer 4 to 50 μm thick is formed on an alloy layer up to 2 μm thick and containing at least one of Ni, Fe, Zn and Sn, and the zinc grain size in the coating layer is defined.
The rust-preventive steel strip for fuel tanks is excellent in various properties necessary for fuel tanks.
Further, Japanese Unexamined Patent Publication (Kokai) No. 8-325692 discloses a rust-preventive steel strip for fuel tanks wherein a coating layer, which is composed of a Sn—Zn alloy and in which the major axis size of the coating metal grains on the uppermost surface is defined, is formed on an alloy layer up to 2.0 μm thick.
The alloy layer of the rust-preventive steel strip for fuel tanks contains at least 0.5% of at least one of Ni, Co and Cu, and the steel strip is excellent in stamping formability, corrosion resistance and weldability, and can withstand soured gasoline over a long period of time.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 9-3658 discloses a rust-preventive steel strip for fuel tanks having a Sn-based alloy coating layer 2.0 to 15.0 μm thick formed on an alloy layer, 0.05 to 1.5 μm thick, containing at least 50% of Fe and Sn, and further having an organic-inorganic composite film formed on the alloy coating layer.
The rust-preventive steel strip for fuel tanks has a Ni plating layer, in a necessary amount, as a substrate plating layer, is excellent in stamping formability, corrosion resistance and weldability, and can withstand soured gasoline over a long period of time as can the rust-preventive steel strip for fuel tanks described in the patent reference 3.
However, suppressing the manifestation of an uneven luster caused by the non-uniformity of the surface properties in the coating layer was not considered in any of the above rust-preventive steel strips for fuel tanks.
Although the uneven luster, especially, does not impair properties of the coating layers such as corrosion resistance and workability, it impairs the appearances and color tone of the steel strips, or the color tone and aesthetic appearance after painting. Therefore, manifestation of uneven luster on the steel strips must be suppressed.
Although manifestation of an uneven luster is considered to depend on the coating methods and coating conditions, no coating method for solving the problem of the uneven luster has ever been proposed.
The specification of U.S. Pat. No. 5,491,036 (published in 1996) discloses a non-lustrous, corrosion-resistant coated steel strip having an ultra-thin intermediate nickel layer applied to the surface of the strip and a coating of two-phase Sn—Zn alloy layer formed thereon by a hot dipping process and a process for producing the same.
The specification further discloses, regarding the formation of the two-phase Sn—Zn alloy layer, the formation of an alloyed layer between the alloy layer and the base steel strip, which results in a strong bond therebetween.
However, no disclosure has ever been made of the relationship between the alloyed layer and the texture of the two-phase Sn—Zn alloy layer that influences the manifestation of uneven luster.
Furthermore, the specification of U.S. Pat. No. 5,491,036 discloses one embodiment wherein coating rollers contacted with the surface of a molten coating bath and holding a coated steel strip from both sides are arranged, and the molten coating layer thickness is controlled and non-coated portions are coated by the rollers during withdrawing the coated steel strip from the molten coating bath. However, means for suppressing the manifestation of the uneven luster has never been disclosed.
As explained above, although users have strictly required the solution of the problem of the uneven luster on Sn-based metal-coated steel strips in recent years, neither the coating methods, nor the coated steel strips that solve the problem of the uneven luster have ever been proposed.